The present invention relates to the field of electronic circuits, and, more particularly, to a current source with a low coefficient of temperature dependence.
A coefficient of temperature dependence is a parameter which, for an electronic device, relates the variations in the device""s output characteristics (i.e., its output current) to the variations in its operating temperature. The operating temperature may be especially influenced by ambient temperature. The temperature dependence coefficient may be defined both for a device in its entirety and for its constituent parts.
The present invention finds applications, for example, in the manufacture of electronic integrated circuits and in circuits including a current source. In particular, the invention may be useful for the manufacture of integrated circuits or circuit components requiring a current source having very little sensitivity to variations in temperature, such as oscillators, for example. Oscillators may be used in portable transceivers that are powered by battery and may be used at highly variable temperatures, for example.
A prior art current source with low temperature dependence is shown in FIG. 1. The current source of FIG. 1 includes a so-called reference current source 10, a bandgap type reference voltage generator 16 that receives a reference current from the reference current source, and a transconductor 18 for converting the reference voltage of the generator 16 into an output current. The current source 10 has two branches 12, 14. These branches provide a reference current which is copied to the reference voltage generator 16 by a double cascoded current mirror 20.
The reference voltage generator 16 includes a resistor 22 connected in series with a bipolar transistor 24 (PNP). The base of this transistor is connected to the collector and to a terminal 26 with a reference potential (e.g., ground). Its emitter is connected to the resistor 22. The voltage Vbg of the generator 16, which is measured between a terminal 25 and the terminal 26, may be expressed in the form Vbg=VEB+R1I. In this expression, VEB is the emitter-base voltage of the transistor 24, R1 is the value of the resistor 22, and I is the value of the current copied by the mirror 20 from the reference current source to the reference voltage generator 16.
The transducer 18 includes an amplifier 27 and of a transistor 28 of the metal-oxide semiconductor (MOS) type. It delivers a current Iout in a load resistor 29 having a value R2 such that Iout=Vbg/R2. Thus, for a bipolar transistor such as the transistor 24, the base-emitter voltage is a negative temperature function (i.e., a negative temperature dependence coefficient). On the other hand, the values R1 and R2 of the resistors 22, 29, as well as the current I copied from the reference generator 10, evolve positively with the temperature.
By appropriately choosing the values of R1 and I and summing the terms VEB and R1I it is possible to obtain, at the terminal 26, a reference voltage generator with a temperature dependence coefficient able to compensate for the temperature drifts of the load resistor 29 and of the transconductor 18. Thus, the output current Iout may be rendered substantially insensitive to temperature. A more comprehensive description of the output source of FIG. 1 may be found in Analysis and Design of Analog Integrated Circuits, Paul R. Gray/Robert G. Meyer, 3rd edition, p. 345 (FIG. 4.50).
The current source of FIG. 1 provides very good temperature stability. Yet, it includes a large number of components and has a high power consumption. These characteristics do not lend themselves to integration of the current source in a high density integrated circuit or reduced circuit cost. Indeed, the chip surface required for such a current source integration is too great for many applications.
Another current source according to the prior art having a smaller number of components is illustrated in FIG. 2. The current source of FIG. 2 combines two individual current sources having opposite thermal behavior. The first individual source 30 is a current source with two branches coupled together by a current mirror. Such a source is known per se and delivers a current that varies in proportion to the temperature. More precisely, the current Ia is such that:       Ia    =                            kT                      qR            a                          ⁢        ln        ⁢                  xe2x80x83                ⁢                              S            2                                S            1                              =                        Δ          ⁢                      xe2x80x83                    ⁢                      V            BE                                    R          a                      ,
where k, T, q, Ra, S1 and S2 respectively represent the Boltzmann constant, the temperature, the electron charge, the value of a source current fixing resistor 34, and the surfaces of emitters of bipolar transistors 31, 32, 33 and 35 (being respectively in two branches of the source). The term xcex94VBE represents a magnitude such that xcex94VBE=(VBE33+VBE32)xe2x88x92(VBE34+VBE31), where VBE33, VBE32, VBE34 and VBE31 respectively indicate the base-emitter voltages of the transistors mentioned above.
The second individual source 40 includes a bipolar transistor 42 connected in series with a current fixing resistor 44 having a value Rb. It is further connected in parallel to the first current source 30. A current Ib delivered by the second source is such that Ib=            I      b        =                  V        BE                    R        b              ,
where VBE is the base-emitter voltage of the bipolar transistor 42. The current Ib is inversely proportional to the temperature, i.e., to       1    T    .
Transistors 51, 52, combined with resistors 53, 54, connect the two sources 30, 40 to a first supply terminal 56, connected to a first potential (Vcc), and to a second supply terminal 58, connected to a second potential (Vee). The transistors 51, 52 have their bases respectively connected to biasing lines 61, 62 which may be used to copy the current of the sources 30, 40 to loads (not shown). That is, they are current mirror control transistors, also not shown.
By adjusting the values Ra and Rb of the current fixing resistors of the two individual sources 30, 40 (and possibly the surfaces of the transistors 31, 32, 33, 35 and 42), it is possible to set the amount of current each current source contributes to the total current passing through the control transistors 51 and 52. It is also possible to set the amount of current each individual source contributes to the thermal drift of the overall source combining the two sources.
Thus, the thermal drifts of the individual sources 30, 40 are respectively proportional to the temperature (positive coefficient) and inversely proportional to the temperature (negative coefficient). As discussed previously, this is due to the fact that one of the sources is of the       Δ    ⁢          xe2x80x83        ⁢          V      BE        R
type and the other source is of the       V    BE    R
type. It is therefore possible to obtain at least a partial compensation for the drifts of the two sources, and therefore an overall source with a low temperature dependence coefficient. A more comprehensive discussion of the current source of FIG. 2 may be found in Evolution of High-Speed Operational Amplifier Architectures by Doug Smith et al., IEEE J. of SSC., Oct. 1994, vol. 29, no. 10.
FIGS. 3, 4 and 5 respectively show the temperature behavior of the first and second individual sources 30, 40 and the overall source resulting from their combination. These figures respectively show, in graphical form, the current (shown on the ordinate) as a function of the temperature (shown on the abscissa). The evolution of the current is given for two different values of the supply voltage (2.7 and 5.5 V) measured between the supply terminals. On each graph, the letters A and B respectively show the curves obtained at 2.7 and 5.5 Volts. The currents are expressed as 10xe2x88x924 A and the temperatures are expressed in xc2x0 C.
It can be seen in FIG. 3 that the curves A and B have a positive slope. This is characteristic of a positive temperature dependence coefficient for the first individual source 30, i.e., the       Δ    ⁢          xe2x80x83        ⁢          V      BE        R
source. On the other hand, FIG. 4 shows a negative temperature dependence of the individual source 40, i.e., the       V    BE    R
source. Temperature drifts of the sources are generally considered to be between xe2x88x9255xc2x0 C. and +125xc2x0 C. compared with an ambient temperature of +27xc2x0 C. Thus, for the first individual source 30, the drift is +33% between xe2x88x9255 and +27xc2x0 C. and +20% between +27xc2x0 C. and +125xc2x0 C., i.e., an overall drift of 53% for a biasing at 2.7 volts.
For the second individual source (FIG. 4), the overall (negative) drift between xe2x88x9255xc2x0 C. and +125xc2x0 C. is xe2x88x9244%, again for a biasing at 2.7 volts. Furthermore, the variation in current at a fixed temperature for a biasing running from 2.7 V to 5.5 V is respectively +30% and +9% for the two individual sources.
In FIG. 5, which gives the temperature behavior for the overall source including the combination of the two individual sources, it may be seen that a bell-shaped evolution of the current as a function of the temperature for a biasing at 2.7 volts is obtained. The overall drift is 24% maximum, i.e., 16% between xe2x88x9255xc2x0 C. and +27xc2x0 C. and xe2x88x9221% between +27xc2x0 C. and +125xc2x0 C. On the other hand, for a supply voltage of 5.5 volts, the bell-shaped behavior disappears and a temperature dependence with a negative coefficient is present. The drift of the overall source is, however, reduced to xe2x88x9236% (xe2x88x9212% from xe2x88x9255xc2x0 C. to +27xc2x0 C. and xe2x88x9224% from 27xc2x0 C. to +125xc2x0 C.).
Compared with the current source of FIG. 1, the current source of FIG. 2 has a smaller number of components and a lower power consumption. On the other hand, its temperature dependence is greater and the quiescent current (at 27xc2x0 C.), just like the temperature dependence coefficient, is very sensitive to the supply voltage.
An object of the invention is to provide a current source having a low temperature dependence while alleviating the limitations of the sources described above.
Another object of the invention is to provide a current source that requires a relatively smaller number of components and is therefore able to occupy a small chip surface when it is part of an integrated circuit.
Still another object of the invention is to provide a current source having a low power consumption and which is less sensitive to variations in its supply voltage.
These and other objects, features, and advantages in accordance with the invention are provided by a current source with low temperature dependence including a reference current source and at least one current mirror to copy the reference current to at least one output branch. The current mirror may be a weighted mirror, and the reference current source and the weighted current mirror may respectively have opposite temperature dependence coefficients. As used herein, a weighted mirror is a mirror which makes it possible to copy in the slave branches (i.e., the output branches) a current which is different and preferably greater than that in the master branch.
As the temperature dependence of the current mirror is opposite that of the reference current source, the temperature dependence coefficient of the overall source (reference+mirror) may be lower than that of the reference current source taken in isolation. Adjusting the characteristics of the reference source and of the mirror thus makes it possible to obtain a very low temperature dependence.
According to the invention, various embodiments may be used for making the reference current source. It may be, for example, a source of the type with a base-emitter voltage reference       (                  V        BE            R        )    .
. Such reference current sources are known in the art and are described, for example, in Analysis and Design of Analog Integrated Circuits, Paul R. Gray/Robert G. Meyer, 3rd edition, p. 324 (FIG. 4.9.a).
In one embodiment of the current source of the invention, a reference source with a negative temperature dependence and a current mirror with positive dependence may be selected. In this case, the positive drift of the current mirror compensates for the negative drift of the reference source when the temperature increases and vice-versa when the temperature decreases. The current mirror may include a first mirror transistor in a master branch connected to the reference current source and at least one second mirror transistor connected in each output branch. The the first transistor may further be connected in series with a weighting resistor.
The current source may include several output branches for the supply of several loads and possibly, as indicated below, to supply the reference current source itself. Indeed, to reduce still further the temperature dependence of the current source, it is possible to supply the reference current source with a supply current substantially insensitive to variations in temperature. Such a current may be provided, for example, by one of the output branches of the current mirror. Such a branch may include a transistor, known as a supply transistor, as one of the second transistors and which forms a current mirror with the first transistor of the master branch.
The weighting resistor makes it possible to obtain a weighted mirror and, in particular, a mirror capable of copying in the output branch (or branches) a current greater than the current provided by the reference current source. A weighted mirror may also be obtained by selecting in the output branch a second transistor with an emitter surface greater than that of the first transistor. By adjusting the value of the weighting resistor or the supply transistor surface, compensation may be made (by way of the mirror) for the variations in source temperature. This is expressed in practice by a mirror copy coefficient greater than 1. A current is therefore available with low sensitivity to temperature and that may be used as discussed above to supply the source via the supply transistor.